Current generation rate responsive implantable pacemakers generally use a programmable, but fixed lower rate limit (LRL). As the term indicates, the lower rate limit establishes a low limit to the pacing rate for a rate responsive pacemaker, to ensure that the pacing rate does not drop to an undesirable brady level. It is desirable to enable a lower pacing rate during nighttime than during daytime, for physiological reasons; during the night, patient metabolism drops and a lower rate is more natural and physiological. Further, as is well known, provision of a lower rate at night means few delivered pacing pulses over an extended period of time, and thus a substantial savings in battery energy. Some current pacemakers provide for automatic drop of LRL at night. Such automatic rate drop at night can be provided simply by an internal clock, or by a detected parameter which indicates low patient activity or metabolism, from which the pacemaker can conclude nighttime.
A problem inherent in the usual form of rate drop at night is that the pacemaker does not follow the patient's circadian variation. Circadian variations may occur due to changes in the individual's state of health or general constitution, as well as summer/winter time transitions and travel to different time zones. For whatever the reason the circadian variations, it is desirable that the pacemaker be able to track and adjust to these variations and changing sleep patterns. Accordingly, what is needed in the art is a pacemaker which is able to continuously sense one or more biological parameters which accurately reflect the patient's circadian variation, and continually adapt the timing for switching between daytime LRL and nighttime LRL to best follow the circadian variations. In the preferred embodiment of this invention, use is made of the observation that daily variations in QT interval (QT) are an accurate reflection of circadian variations. At nighttime, the value of QT rises toward a daily maximum, and recognition of this enables a determination of patient nighttime suitable for lowering LRL.